A quasi-phase matched wavelength conversion optical element has been proposed in which a periodic polarization inversion structure is realized by applying a stress to quartz crystal (SiO2), which is a paraelectric material, in the vicinity of the α-β phase transition temperature, so that a periodic twin structure is created (S. Kurimura, R. Batchko, J. Mansell, R. Route, M. Fejer and R. Byer: 1998 Spring Meeting of the Japan Society of Applied Physics Proceedings 28a-SG-18). This is a method in which a quasi-phase matched crystal based on quartz is manufactured by utilizing the twin crystal of quartz to achieve a periodic inversion of the sign of the nonlinear optical constant d11, and this quasi-phase matched crystal is used as the quasi-phase matched wavelength conversion optical element.
In the case of quartz, the short absorption edge is a wavelength of approximately 150 nm, and ultraviolet absorption at wavelengths shorter than 200 nm is almost negligible compared to the case of nonlinear optical elements using conventional birefringence phase matching (β-BaB2O4 and CsLiB6O10, etc.) or nonlinear optical elements using the quasi-phase matching of ferroelectric materials (LiNbO3 and LiTaO3, etc.). Furthermore, this also has the following special feature not seen in conventional nonlinear optical elements: namely, there is a possibility of generating, with a high efficiency, light with a wavelength of approximately 193 nm, which is comparable to the light of an ArF excimer laser, by second harmonic generation. By setting the period at several microns to several tens of microns, it is possible to use the twin structure as a practical wavelength conversion device. A semiconductor exposure apparatus using this quasi-phase matched quartz crystal has been proposed (Japanese Patent Application Kokai No. H2002-122898.
Initially, a method in which a periodic Cr film is formed on the surface of a quartz crystal substrate using lithographic and thin film formation techniques, and the temperature is elevated to around 550° C., so that thermally-induced stress is applied utilizing the in-plane stress arising from the difference in linear thermal expansion coefficient between the quartz crystal and Cr, was proposed as a method for manufacturing a periodic twin structure in a quartz crystal (S. Kurimura, R. Batchko, J. Mansell, R. Route, M. Fejer and R. Byer: 1998 Spring Meeting of the Japan Society of Applied Physics Proceedings 28a-SG-18). In this method, the crystal axes are inverted only in the portion where the Cr film is formed, so that a periodic twin structure is produced.
In this method, however, since a stress component that is parallel to the plane of the quartz crystal substrate is used, stress is localized in the vicinity of the surface of the crystal, so that a portion with inverted crystal axes is formed only to a depth of a few microns from the surface of the crystal. Consequently, since a periodic twin structure which extends into the interior portions, i.e., a periodic twin structure with a large aspect ratio, is not obtained, the realization of a bulk optical element that can be used for the wavelength conversion of laser light is impossible.
The hot pressing method has been proposed as the method for solving such problems (S. Kurimura, I. Shoji, T. Taira, M. Fejer, Y. Uesu and H. Nakajima: 2000 Fall Meeting of the Japan Society of Applied Physics Proceedings 3a-Q-1) In this method, a periodic step structure is formed on the surface of one side of a quartz crystal substrate, this quartz crystal substrate is clamped between heater blocks from above and below, the temperature of the quartz crystal substrate is elevated, and pressure is applied at the point in time at which this temperature reaches a desired temperature. In this case, since stress acts only on the portions corresponding to the protruding parts of the step structure, the crystal axis is inverted only in these portions. These portions with inverted crystal axes grow to the interior of the crystal and are thus propagated into the crystal, so that a periodic twin lattice that penetrates greatly in the direction of depth can be manufactured. Specifically, stress is concentrated only in the protruding portions, and twins are generated from these areas; these twins gradually grow into the interior, so that a twin structure with a large aspect ratio is manufactured.
An example of such a quartz crystal substrate in which a step structure is formed on the surface of one side will be described with reference to FIG. 4. A step structure is formed on the surface of one side of a quartz crystal substrate 1; as a result, protruding parts 2 that have a solid rectangular shape are formed at a specified interval. These protruding parts 2 have a specified width in the left-right direction in the figure, and the plurality of protruding parts 2 are formed at a spacing that is the same as this width in the left-right direction in the figure. The protruding parts 2 have a rectangular solid shape that is long in the direction of depth in the figure. The direction parallel to this longitudinal direction is taken as the direction of the a axis. Naturally, the a axis is perpendicular to the normal of the quartz crystal substrate 1.
The direction of the c axis of the quartz crystal is perpendicular to the a axis; however, this c axis is slightly inclined with respect to the normal direction of the quartz crystal substrate 1, as described by S. Kurimura in the May 2000 issue of the Journal of the Japan Society of Applied Physics. Specifically, the quartz crystal substrate 1 is cut so that the normal and the c axis form a slight angle. The reason for this is that the component of an elastic compliance tensor s1123 is used in a procedure in which a periodic twin structure is manufactured by the stress application of the inter-twin energy difference. Twins tend to be generated as this angle increases; in actuality, however, this angle is kept to an angle of approximately 10 to 20 degrees.
As is shown in the figure, the light is incident from the end surface of the quartz crystal substrate 1, and the direction of polarization is the same direction as the direction of the a axis. The light whose wavelength is converted inside the quartz crystal substrate 1 is emitted from the end surface that is located on the opposite end from the incident surface.
An example of the method for manufacturing the protruding parts 2 shown in FIG. 4 will be described below. First, a Cr film is formed to a thickness of approximately 100 nm on the surface of the quartz crystal substrate 1 by a sputtering method. The surface of this film is coated with a positive type resist, and the portions other than the portions that will form the protruding parts 2 are exposed and developed using a semiconductor exposure apparatus such as an i-line stepper. Next, the Cr film is fused and removed using the remaining resist as a mask. Then, wet etching is performed by means of hydrofluoric acid using the remaining resist film and Cr film as a mask, so that a step structure with a depth of about a few microns is manufactured. As a result, a quartz crystal substrate 1 is completed which has a step structure with protruding parts 2 such as those shown in FIG. 4 formed on the surface. Furthermore, the Cr film may either be stripped or not stripped prior to pressing.
It is convenient in a wavelength conversion device that the angle between the c axis and the normal of the quartz crystal substrate be as small as possible; however, the stress required for twin formation is increased in this case. Accordingly, as was described above, the quartz crystal substrate is cut so that the angle between the c axis and the normal of the quartz crystal substrate is an angle ranging from a few degrees to approximately 20 degrees.
FIG. 5 shows a schematic diagram of an example of a pressing apparatus that applies temperature and pressure to such a quartz crystal substrate. This is basically a pressing apparatus comprising four supporting columns 41, an upper block 42 and a lower block 43. The upper block 42 is attached to an upper plate 44, and the upper plate 44 is fastened in place by being clamped by nuts from both sides of the supporting columns 41. The lower block 43 is attached to a lower plate 45, and the lower plate 45 is mounted on a hydraulic cylinder 46. The lower plate 45 and lower block 43 are caused to move upward and downward by the raising and lowering of the piston of the hydraulic cylinder 46, so that a pressing force is generated. A load gauge 47 is disposed between the hydraulic cylinder 46 and lower plate 45, so that the value of the applied load can be monitored.
Respective heaters 48 are attached to the upper block 42 and lower block 43, and are covered by an adiabatic plate (e.g., Hemisul (trademark)) except for the pressing surfaces. The upper block 42 and lower block 43 are respectively made of SUS304, and the pressing surfaces of these blocks are precisely polished to a flatness of approximately 3 μm. Holes are formed in the upper block 42 and lower block 43 from the side surfaces, and the heaters 48 are inserted into these holes. Furthermore, thermocouples (not shown in the figure) used for temperature control are inserted into the side surfaces of the upper block 42 and lower block 43, and these thermocouples are connected to a temperature adjustment device (not shown in the figure). This temperature adjustment device is constructed so as to operate the current that flows through the coils of the heaters 48 by PID control; as a result, the pressing surfaces of the upper block 42 and lower block 43 can be elevated to a desired temperature.
A combination of air pressure and hydraulic pressure is used to control the pressing load. Air at a pressure of several hundred MPa is supplied to an electrical air regulator 49 which can control the flow rate by means of voltage. A voltage from a function generator 50 that can generate arbitrary voltage patterns with respect to time is supplied to the electrical air regulator 49.
The air that is generated by this electrical air regulator 49 is supplied to a hydraulic pressure conversion amplifier 51. This hydraulic pressure conversion amplifier 51 has a 36-fold pressure amplification function. The hydraulic pressure of the hydraulic pressure conversion amplifier 51 is converted into a load by being supplied to the hydraulic cylinder 46, so that a maximum pressing load of approximately 10 kN can be obtained.
A quartz crystal substrate 52 of the type shown in FIG. 4 is placed on top of the lower block 43. In a state in which the lower plate 45 and lower block 43 are raised so that the quartz crystal substrate 52 is clamped by the upper block 42 and lower block 43, the temperature is elevated to the vicinity of the twin transition temperature of the quartz crystal; then, when the desired temperature has been reached, a stress is generated in the surface of the quartz crystal by applying the pressing load described above to the surface of the quartz crystal substrate 52. A desired stress waveform can be obtained in the tracking ranges of the air pressure and hydraulic systems described above by varying the voltage pattern that is generated by the voltage generating device 50.
The quartz crystal substrate 52 has the structure described above, and has a size ranging from several millimeters to several tens of millimeters. Furthermore, after twins are generated at one time by using a wafer with a diameter of several inches or so, these twins can also be cut into smaller segments. The thickness of the quartz crystal substrate 52 is set at approximately 0.1 millimeters to several millimeters so that utilization as a bulk optical element is possible.
Both surfaces of the quartz crystal substrate 52 are polished with high precision, thus ensuring the degree of parallel orientation of both surfaces of the substrate and the surface precision of the respective surfaces so that the mean wave aberration is approximately 0.03λ (λ=0.638 μm) or less. In the case of quartz crystal, this corresponds to a surface precision of approximately 0.045 μm, which is sufficiently fine compared to the steps formed in the quartz crystal surface described above.
Conventionally, as was described above, wet etching by means of hydrofluoric acid has been used as a method of forming steps in quartz crystal. However, in the case of wet etching using hydrofluoric acid, the side surfaces are also removed in addition to etching in the direction of depth; accordingly, especially in cases where a twin structure with a short period is formed, steps are not formed with the same period as the mask. Furthermore, since etching proceeds to a greater extent in the direction oriented along the c axis, the following problem also arises: namely, the depth that is removed varies according to the angle of the c axis with respect to the normal of the quartz crystal substrate.
Furthermore, in the case of a pressing apparatus such as that shown in FIG. 5, the upper pressing surface and lower pressing surface must be parallel in order to apply stress in a uniform manner to the quartz crystal substrate in which both surfaces are being planarized more or less parallel to each other. Moreover, the surface flatness of the respective pressing surfaces must be sufficiently fine compared to the step depth (about a few microns) on the surface of the quartz crystal substrate.
In the case of the conventional pressing apparatus shown in FIG. 5, the lower plate 5 to which the lower block 43 is attached slides along the supporting columns 41, so that the pressing surface of the lower block 43 cannot be adjusted. Accordingly, in order to adjust the degree of parallel orientation of the upper pressing surface and lower pressing surface, it is necessary to move the upper plate 44 to which the upper block 42 is attached. The upper plate 44 is fastened to the four supporting columns 41 (in which screw threads are cut) by being clamped by nuts. An adjustment is performed by turning these nuts so that the pressing surface of the upper block 42 and the pressing surface of the lower block 43 are oriented parallel to each other.
However, adjustment so that the pressing surface of the upper block 42 and the pressing surface of the lower block 43 maintain a degree of parallel orientation of a few microns is extremely difficult, since actual screws have play and backlash. Furthermore, even assuming that such an adjustment is favorably accomplished, the following problem is encountered: namely, loosening of the screws, etc., in repeated pressing makes it impossible to maintain the degree of parallel orientation, so that re-adjustment must be performed for each operation. For such reasons, the distribution of the load that is applied to the surface of the quartz crystal substrate becomes non-uniform, and this makes it difficult to manufacture a periodic twin structure in a uniform manner.
Furthermore, the surface flatness of conventionally used pressing surfaces is approximately 3 μm, which is more or less the same as the depth of the step working in the surface of the quartz crystal; accordingly, it is difficult to say that the load is uniformly applied.
Moreover, conventionally used pressing surfaces are formed from SUS304; this material tends to soften at high temperatures, and is therefore unsuitable for use in the formation of twins by applying a stress to the surface of the quartz crystal substrate. Furthermore, other problems such as the following problem also occur: namely, traces corresponding to the protruding parts of the step working of the quartz crystal remain in the pressing surfaces after pressing, so that the surface flatness deteriorates. Thus, in conventional pressing apparatuses, there are problems not only in terms of the degree of parallel orientation of the pressing surfaces, but also in terms of the surface flatness and material of the pressing surfaces.
Problems have also been encountered in the pressure control method used in conventional pressing apparatuses. First of all, in the case of control by hydraulic pressure using a hydraulic pressure conversion amplifier, the drawback of a slow response speed is a problem. Specifically, the response time of air pressure is a maximum of 100 ms or less; however, since the rise time of a hydraulic pressure conversion amplifier is long, a time of approximately 1 second is required for the transition from a state in which the load is 0 kN to the maximum load of 10 kN. Furthermore, there is a dead time of 100 ms or longer from the input of the voltage signal to the response of the hydraulic pressure.
Furthermore, in the case of control by hydraulic pressure, the problem of a narrow load setting range arises. In the hydraulic pressure conversion amplifier described above, in cases where a very small air pressure is applied, there is a failure to respond because of the viscosity of the oil. Accordingly, in a system with a maximum load of 10 kN, it is difficult to generate a load of approximately 2 kN or less. Consequently, in cases where the optimal load for twin formation is 2 kN or less, the hydraulic pressure conversion amplifier must be replaced. In such cases, all of the oil in the hydraulic system must be removed and replaced, so that an extremely large amount of work is required; moreover, specialized expertise is required. Furthermore, when the oil is injected following replacement of the hydraulic cylinder, care must be taken to ensure that no air is admixed with the oil, so that considerable time is required. For such reasons, it is extremely difficult to find optimal load conditions for the manufacture of a periodic twin structure in the case of conventional pressing apparatuses.
Furthermore, in a conventional pressing apparatus, as is shown in FIG. 5, both the upper block 42 and lower block 43 each use only one heater 48; consequently, the distance between the heater and the pressing surface varies according to the location on the pressing surface. As a result, a temperature variation with a maximum value of several tens of degrees Celsius is generated in the pressing surfaces, so that the following problem arises: namely, a temperature distribution is produced in the quartz crystal substrate as well, and even if a stress is uniformly applied, there is some irregularity in the manner in which the twin is formed.
Furthermore, in cases where a twin is formed in a quartz crystal using a conventional hot pressing method, the following problem arises: namely, as was described above, the work of forming a step structure on one side of the quartz crystal substrate using photolithographic and wet etching methods must be performed for each quartz crystal substrate.
The present invention was devised in light of the above circumstances; it is an object of the present invention to provide a quartz crystal substrate and a pressing apparatus that make it possible to solve the various problems described above.